Here's my take on the subject. I'll dance around it (The Oktoberfest Chicken Dance).
When my wort is near boiling I will turn up the heat and skim off the foam and let it come to near boilover, hard rolling boil & skim some more. To me that is vigorous. Then after I am sure the hot break has occured, I turn down my heat to a nice rolling boil and at that point I add my first hop addition and start my time for a 60 minute boil.

I have a 15 gallon keg conversion that I use as a boiling kettle. My evaporation rate around 22%. I want to start doing 10 gallon batches, but with my rate that high I don't have the space in the kettle to collect 11.5 gallons that I will need- 10 gallons plus 1.5 gallons deadspace. I have a propane burner that I have set rather high. Does anyone know of a minumum evaporation rate? Or a better means of gauging a boil? I really don't want to have to make a giant pressure cooker. One way would be to put a lid on the kettle, but I don't think that's wise because some compounds need to be boiled out, but then again, would there come a time when they should be boiled off and I could put a lid on?

I will give some of my observations and hopefully someone else can either dispute or verify.

ProMash uses 5% evap rate as default. I think Charlie Papazian says average of 10%. I get about 7%. I boil vigorously, uncovered, (for about 15 minutes) until after first wort hopping then turn down the heat to a rolling boil (for 60 minute boil, total of 75 minutes) and have the lid on part way (1/2 way?) depending on the weather and other conditions. (Evaporized & condensed wort compounds should not be allowed to drip back into the boil from off of the lid.) I boil in a 15.5 keg also and boil 14 gallons to get about 12+ gallons into the fermentors, which includes some trub loss but not much in deadspace. I use a 115K BTU turkey fryer and looking at my notes I got 4 -5 gallon batches and 4 -10 Gallon batches then went to refill my propane tank and it was still about 1/3 full.

I just read another forum where a Homebrewer ran out of gas after only partway into his 4th batch!

My system is a very similar setup. 10 gallon batches in a 15.5 gal converted keg.

I seem to lose about 15% to the boil and cooling, so I start with 12 gallons in the kettle (yes, this almost always equals a boilover if I'm not meticulously positioned with a water-bottle and stainless stirring spoon).

After a 75 minute boil I have two 6.5 gallon glass carboys filled to just under their "shoulder" and a perfect rack into 5 gal carboys with almost no air in the neck for secondary.